Generally, wireless radio communication systems are employed to connect mobile radio/telephone users together. Implementation of such systems, also known as "cellular" telephone services or Personal Communication Services "PCS," has become commonplace, particularly in metropolitan areas. As shown in FIG. 1, typical system configurations divide up a particular geographic service area 10 into several smaller geographic cell sites 12. Communication within each cell site is provided by a fixed base station 14, and communication between the cell sites or between a cell site and a land based telephone network is controlled by a control call processor 16 located in a mobile telephone switching center 18. The mobile telephone switching center 18 is connected to a public switched telephone network 20.
The respective boundaries of each cell site 12 are generally defined by the transmission and reception range of the respective base stations. These cell boundaries typically overlap at adjoining edges to enable communication throughout the total service area by permitting mobile users 22 to move among the different cells without disrupting communication capabilities, i.e., cell site hand-offs.
In order to allow a large number of users to have simultaneous access to an otherwise limited band-width, typical cellular systems employ either frequency division multiple access (FDMA), time division multiple access (TDMA), or code division multiple access (CDMA) as a system access protocol. Of particular concern to the present invention is a CDMA protocol.
In contrast to FDMA and TDMA, CDMA allows multiple users to simultaneously share all time and frequency resources of the system. This is generally achieved by assigning each user a unique pseudo-random code or identification code which is then used during modulation and demodulation of transmitted signals. An identical code is used at the other end of the transmission to only allow receipt of data that has a matching code sequence. Because the unique code effectively spreads the spectrum of the transmitted signal beyond that which is needed for transmitting the baseband signal, CDMA facilitates rejection of unwanted signals, thereby enhancing system performance in highly noisy environments.
Each base station employs an antenna system arranged to be capable of communicating signals throughout a particular cell's coverage area. One important aspect in the operation of a base station antenna system is to optimize of the strength of signals communicated with mobiles located at the outer boundary. With respect to signals transmitted from the base station to a mobile user, i.e., a downlink channel, weakness of the signal can be overcome merely by increasing the level of power for the base station's transmitter. However, reliable base station reception of signals sent by mobile users, i.e., via an uplink channel, are more problematic due to typical low gain designs of conventional mobile transceivers.
In addition, because of the inherently noisy environment and potential for multi-path signal fading, known CDMA base station antenna systems generally employ spatial or frequency diversity as a mechanism for improving transmission reliability. More specifically, with spatial diversity, the antenna system includes at least two downlink antennas physically separated either horizontally or vertically by an appreciable number of wavelengths. The respective outputs of the antennas are supplied to a processor for determining which antenna is receiving the best quality signal. The best signal path is subsequently connected to the receiver subsystem of the base station. With a wideband signal like that used in CDMA systems, different frequencies will selectively fade in the 1.25 Mhz channel. This frequency diversity arrangement mitigates signal outages due the effects of Rayleigh or "fast" fading.
While spatial and frequency diversity arrangements improve uplink channel performance, a need still exists for a base station antenna architecture which improves uplink performance, particularly in congested metropolitan areas. While it is possible to provide higher gain simply by increasing the size of the respective antennas, such a solution may not be feasible due to physical space constraints or cost.